This invention relates to a bulky and highly soft and shock-absorbing nonwoven fabric and a method of manufacturing the same.
Melt-blow type nonwoven fabrics produced by collectively bonding extremely fine thermoplastic resin filaments that have the toughness of fibers have been known and used typically for wipers and other applications.
Dry laid nonwoven fabrics produced by collectively bonding thermoplastic resin filaments that appear and feel like fibers have also been known and used mainly for top-layer seats of diapers.
Meanwhile, Japanese Patent Publication No. 57-17081 (U.S. patent application Ser. No. 535,003/1974) discloses an absorbing structure prepared by disposing a surface sheet of resin film on a sheet of absorbing material. Said resin film is provided with a large number of small circular holes per unit area, each of which holes carries a tapered capillary tube arranged around its periphery, which capillary tube penetrates into the inside of the absorbing material through its surface. An absorbing structure as described above has applications in diapers, sanitary napkins and lining pads of beds.
Japanese Patent Disclosure No. 57-19311 (U.S. patent application Ser. No. 230,488/1981) teaches a plastic film or web provided with evenly and densely distributed holes.
Japanese Patent Disclosure No. 64-64655 (DE Patent Application No. P 37 23 404. August 1987) teaches a plastic film having a large number of small circular holes formed on it as it is inflated and burst. The tiny debris of the burst sticking to the peripheries of the holes as so many projections.
Japanese Patent Disclosure No. 64-72745 discloses a sheet of nonwoven fabric, which is suitably used for a top-layer sheet of an absorbing item such as a diaper.
According to the teaching of the invention, the sheet forms a second layer of porous and water-repellent material which is fitted onto a first layer of aggregated filaments that comes to contact with the skin surface of the user. With such an arrangement, such a sheet may be suitably used as a top-layer sheet of an absorbing item.
Said Japanese patent disclosure also teaches a first and a second method of combining such a porous sheet with a nonwoven fabric, of which the first method comprises a step of preparing a nonwoven fabric and a porous sheet separately and a step of bonding them together, whereas, according to the second method, a fabric web is placed on a porous sheet and then the fabric web is rigidly bonded to the porous sheet to form an integrated item.
A melt-blow type nonwoven fabric is required to be soft, bulky and shock-absorbing when it is used for a shock-absorbing item or a wiper.
While a melt-blow type nonwoven fabric is normally flexible and soft, it has a poor gas and water permeability, making itself unsuitable for applications such as the top-layer sheet of a paper diaper which is required to immediately pass the discharged urine to the absorbing layer and hence should have a high water permeability.
Moreover, a nonwoven fabric should be highly soft, shock-absorbing and at the same time permeable to both water and gas particularly if it is used for a medical care item for covering a wound, a baby diaper which is required to be free from causing rashes on the skin or a bed pad for a patient staying in bed for a prolonged period of time without causing any sore skin.
Thus, top-layer sheets disclosed by the Japanese Patent Disclosures Nos. 57-17081, 57-193311 and 64-64655 cannot satisfactorily meet the above requirements because they are made of film and, although they are to some extent permeable to water and gas, give a chilly feeling, to say nothing of their insufficient softness that makes them undesirable to be brought into contact with the skin.
Although a nonwoven fabric can be made permeable to water and gas by boring small holes through it, a significant portion of the nonwoven fabric is wasted when holes are bored by mechanical means particularly when the holes are densely distributed throughout the fabric. Therefore, such a method of boring holes is unrealistic and economically not feasible. If the holes are bored by having the fabric pierced with needles, the periphery of each of the holes can be molten and then hardened and the hardened areas immediately lose their softness. Moreover, the fabric does not necessarily become soft by simply forming small holes running therethrough.
With a known top-layer sheet having a layer of non-woven fabric, since the holes of the surface layer are covered by the nonwoven fabric, its water permeability is inevitably defined by the water permeability of the non-woven fabric. Moreover, it is obtained by simply laying a porous surface sheet on a non-woven fabric, it cannot provide a satisfactory elasticity nor a sufficient buffering property.
U.S. Pat. No. 4,041,951 also discloses nonwoven fabric for similar applications. It shows a disposable diaper having a substantially planar, moisture absorbent layer disposed between a soft and bulky, wearer-contacting top sheet which is uniformly moisture pervious along its entire surface and a moisture resistant backing sheet. The top sheet is comprised of a generally hydrophobic non-woven material and has depressed areas and non-depressed areas. The depressed areas are comprised of embossments and contact the uppermost surface of the substantially planar, moisture absorbent layer in use. The non-depressed areas contact the wearer's skin in-use.
With such a configuration, however, since it shows significant unevenness because of the embosses, the projecting sections become less soft without forming bulky cylindrical projections, making the touch of the top sheet less comfortable.